The present invention relates to a control system for a digital typesetter for imaging graphic quality characters of a digital font stored in machine memory.
A digital typesetter images typographic quality characters coded in digital form and stored on a medium such as a magnetic tape, drum or rigid or floppy disc. Such digital typesetters are normally provided with a cathode ray tube (CRT) or laser beam imaging system for writing characters onto photographic film or charged photosensitive surface. In the case of digital CRT typesetters, successive characters are each completely imaged, one at a time by a CRT beam. Normally, each character is built up from a series of vertical strokes which commence from the left side bearing of the character and proceed successively to the right side bearing. Once a character has been completely stroked, the CRT beam is moved to the left side bearing position of the next successive character to form the next character.
Where a laser beam rather than a CRT imaging system is used to reproduce characters, a modulated one dimensional scanning beam has generally been used. The character writing beam is swept across the entire width of the output film or paper in a single line passing through many characters and the print medium is moved in a direction transverse to the direction of the scan line. While such a laser-based imaging system offers advantages over a cathode ray tube system, such a system requires computation of the beam switch points to turn the scanning beam on and off with sufficient speed to keep up with the scanner on a real time basis. In order to overcome this disadvantage, the machine employs a single type size to be set. In this way, the encoded character information may be used directly without scaling, which would require extensive computation. The disadvantage of this arrangement is that an entire font of characters must be stored in the machine for each of the desired type sizes. This results in excessive use of storage space within the machine or requires that the machine be repeatedly "dressed" with a different digitally encoded font each time a different type size, or a different font are to be used.
Within these prior art systems, an imaging surface is carried on a moving support surface such that the beam of modulated light is scanned across the surface in a direction substantially transverse to its direction of travel resulting the well-known raster type scan. As in the known prior art, the imaging surface may be located on a belt which travels between charging stations. Within known prior art devices, the printing operation from charging to stripping takes place in one cycle of the medium.
Other prior art devices place the imaging surface on a drum, either in the form of a paper sheet placed on the drum, or where the photosensitive surface is part of the drum and the image is subsequently transferred to a sheet. As in the case of the belt, the reproduction process takes place in one cycle of the drum wherein the photosensitive surface is changed, imaged, developed and stripped.
These prior art systems typically employ timing means such as marks located on the belt or on the drum to synchronize the operation of the reproduction stations to the position of the drum or the position of the belt. These timing signals are then used to control the operating stations of the recording unit and to synchronize the output of data with the position of drum or belt.